The posts on this and the previous string, plus a little more research, show three schools of thought.
One is that the term ‘Mk’ need not be applied to Thames Ditton Cobras at all. This is the line taken by AC and Shelby American in period, where it was never used in the marketing of any of their products, and by the SAAC and ACOC period Cobra registrars.
The second, widespread, is based on classifying by AC chassis engineering changes:
MkI = leaf spring worm & sector MkII = leaf spring rack & pinion MkIII = coil spring
The third, much less common, is based on classifying by Ford engine model: MkI = 260 (leaf spring) MkII = 289 (leaf spring) MkIII = 427 (coil spring)MkIII/289 = 289 (coil spring) There are variants to this less common school: some, for example, say that a MkIII is 427 and that an AC 289 is an AC 289 (no Mk designation), others that a MkIII is a coil spring 289 and that a 427 is a 427 (no Mk designation).
The widespread AC chassis engineering change classification carries much greater authority than the engine model classification. The engineering designation was agreed by John Rimer (ACOC Cobra Registrar) and Bill Kemper in 1971 and used in the earliest SAAC and ACOC Cobra registers.
The earliest knowledgeable use I have of the less common Ford engine model classification is in Trevor Legate’s 1984 ‘Cobra’ (p48, 104, 124 etc). This classification leads to rather odd conclusions, such as 8% of MkIIs having worm & sector steering & fabricated front uprights and 92% having fundamentally different arrangements.
Ned Scudder and I avoid all these complications by classifying the SAAC period and ACOC Thames Ditton Cobra registers into Leaf Spring Cobras and Coil Spring Cobras (Shelby and AC kept separate); neither register uses the ‘Mk’ word in any car or model description.
The simplicity and clarity of the terms leaf spring & coil spring, worm and sector & rack and pinion, 260, 289 & 427 lead me to suggest that the term ‘Mark’ is best not applied to Thames Ditton Cobras at all. It should be reserved for products that are branded ‘Mk’ or ‘Mark’ by their manufacturers. Neither AC Thames Ditton nor Shelby American in period ever used Mk as a brand; however in later times Angliss, Lubinsky, Gullwing and Superformance have all use Mk as a marketing brand.